The invention is directed to a process for the production of high density U.sub.3 O.sub.8 sintered fuel grains of fixed granulation as fuel for the manufacture of plate shaped fuel elements for material testing and research reactors from fine grained and still sinterable U.sub.3 O.sub.8 powder produced according to known processes. The true fuel plates in this case contains the U.sub.3 O.sub.8 fuel bound in an aluminum matrix.
The fuel element of the classical material test-reactor (MTR) type is constructed of level or bent fuel plates. Each individual plate represents a stratified body whereby the true fuel is encased with material retaining aluminum cladding on all sides.
The fuel plates generally have a thickness of 1.27 mm and a width of about 72 mm. The true fuel zone--the "meat"--thereby has a thickness of 0.51 mm and a width of about 63 mm; the active length is, e.g. 600 mm.
The MTR elements of this construction are distinguished by a desired large heat transferring surface.
The production of fuel plates normally takes place by plating rolls according to the so-called "picture frame technique". As fuel there has previously proven good in industrial use aluminum-uranium-alloys and uranium compounds dispersed in aluminum matrix.
Hereby preferably uranium aluminize and uranium oxide in the form of U.sub.3 O.sub.8 are used as uranium compounds. The latter fuels are distinguished by being able to be inserted successfully in reactors of high power density (2000 kw/l, e.g. HFIR-Oak Ridge, USA) up to high burn-up because of their high loading capacity. In using U.sub.3 O.sub.8 and uranium aluminide there were produced in the maximum output of the reactor core burn up of 2.times.10.sup.21 fissions/cm.sup.3 meat, which corresponds at highly enriched uranium (93% U235) to about 70% burn-up (IAEA-Guidebook, December 1979, ORNL-4856).
The use of uranium with high enrichment of the isotope U.sup.235 as fuel therefore has not ultimately proven good for MTR fuel elements because in so doing the necessarily high nuclear fuel inventory can be produced at relatively low fuel density in a simple manner. On the other hand uranium of high U.sup.235 enrichment represents sensitive materials whose propagation should be well controlled and should be limited. Because of the proliferation therefore there exists the demand to be able to also add uranium of low U.sup.235 entrichment (maximal 20 weight %) for the MTR reactors. The conversion of the MTR reactors to low U.sup.235 enrichment depends on the nuclear fuel inventory being increased for compensation of the increased neutron loss through the higher U.sup.235 content. This and the insertion of uranium of low enrichment requires substantially higher fuel density at unchanged plate geometry. For this U.sub.3 O.sub.8 with its relatively high uranium density of 7.1 g U/cm.sup.3 from the fuel supplied for the MTR reactors is especially well suited because of its sufficiently good compatibility with aluminum.
In order to produce the relatively high uranium densities in meat of more than 2.6 g U/cm.sup.3 high requirements are placed on the granulation of the fuel.
The fuel particles must have a high strength in order to form no segragates in rolling the plates, The open porosity of the granules must be as low as possible in order that the gaseous fission products be retained in the fuel itself and that no inadmissibly high compression stress of the plate occurs. Besides there is required a good embedding of the fuel granules in the aluminum matrix. This assumes that the fuel particles are present in specific, narrowly tolerated particle sizes.
With the previously known process for the production of U.sub.3 O.sub.8 fuel powder for MTR reactors the mentioned requirement are not guaranteed in sufficient measure. The fuel granules produced according to the previous process are constructed of individually agglomerated particles in their macro-structure. The thus structured fuel particles are destroyed in the rolling step and there is formed undesired segragatt which leads to inhomogeneous fuel distribution. Besides the material retaining embedding of the fuel in the matrix is not guaranteed and therewith the mechanical integrity of the fuel plate is not guaranteed.
Therefore it was the problem of the present invention to develop a process for the production of high density U.sub.3 O.sub.8 sintered fuel grains of fixed granulation as fuel for the manufacture of plate shaped fuel elements for material test and research reactors from fine grained and still sinterable U.sub.3 O.sub.8 pwoder produced according to known processes, which leads to a fuel of high density, high strength, and low open porosity with high ability for retaining the fission gases formed in the radiation and which is suited for the further processing by plating rolls.